1. Field of the Invention
This invention relates generally to a communication network line protocol, and more particularly to a data channel or line protocol for providing the exchange of serialized digital signal information between electronic processing units.
2. Description of the Prior Art
There are many applications where it is either desirable, or necessary to serially communicate digital information between electronic devices over a single communication channel. The type of coupling for such a communication channel can be either light, magnetic, RF carrier, or electrical. Regardless of the type of coupling used, the communication link or line protocol plays a very important and determinate role in the overall system equipment design.
There are many well known serial line protocols for controlling the transfer of digital signal information between electronic data processing units. These line protocols are required to organize the transfer of data from one processing unit to another in a manner which assures correct sequening and data integrity. All known types of serial line protocols each have particular characteristics and advantages related to the type of interface over which data is transferred, i.e. single or dual channel, full or half-duplex, synchronous or asynchronous, etc.
All known serial line protocols have a common characteristic, each requires a significant number of dedicated, overhead signal bits within the data or message format (i.e. signal bits that are required for use other than for the transfer of data or message information). These bits are required to ensure the accuracy of the data transmitted in the information field of a message block. Typical overhead bits in a message block comprise the header filled positions (start flag, address field and control field) and trailer field (frame checking and stop flag). Each field (header and trailer) may comprise twenty-four bits for a total of forty-eight bits in each frame. These dedicated signal bits are an overhead cost in the transmission of data under the particular line protocol format since they require some portion of the line utilization time period which may otherwise be used for the transfer of data. This results in some degree of loss in throughput efficiency of the line.
The above discussed overhead bits become particularly undesirable in communication systems where relatively short message lengths (eg. one hundred or less bits per message unit) are interspersed with relatively long pause intervals between the messages or even between the data bits of a message. In this type of communication system, the required protocol format is essentially a link control format which must substantially reduce the bit overhead to permit a high data throughput while still allowing for the highest accuracy possible, i.e. intercommunication integrity. In such a communication system, erratic or faulty transmissions could result in catastrophic failure in either of the communicating processing units. Further, such a communication system does not permit for absolute synchronous data transmission since the nature of the system installation is that of remotely located or detachable processing units where the use of separate clock lines between the units is impractical and sometimes impossible. Therefore, the protocol must ensure low overhead to allow for the highest throughput rates, error control to ensure the highest accuracy to avoid any catastrophic failures, and provide timing for the precise exchange of digital signal information between processing units, regardless of the data bit rate of transmission, whether it be constant or variable.